PROSE+ project: offshore seismic measurements on the seabed to test the ability to assess the spatial variation of the small-strain shear modulus in the subsurface environment

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4. PROSE+ project: offshore seismic measurements on the seabed to test the ability to assess the spatial variation of the small-strain shear modulus in the subsurface environment

The increase in the number of offshore wind farm siting projects, combined with the multiplicity of developments in anchoring techniques, means that foundations and anchors need to be optimized for the conditions of the offshore subsurface, requiring precise knowledge of the mechanical characteristics of the medium. According to the recommendations of the CFMS (French committee of Soils Mechanics), detailed geophysical reconnaissance is then necessary during the project phase (design and execution) to obtain the most accurate information possible at the locations of the structures. In this context, the PROSE+ project aims to increase knowledge and provide new methodological and technical elements, based on surface seismic and geoelectrical techniques. This will make possible to approach heterogeneous environments in a quantitative and non-destructive way, thereby reducing the number of costly and invasive geotechnical surveys. To this end, we developed numerically a 2D seismic inversion technique using Surface Seismic Waves based on Particle Swarm optimization methods. In order to validate it on experimental data, we carried out measurements off Concarneau using 70 4-components sensors (GPR - Sercel nodes) placed on the seabed, in an unprecedented manner. 241 seismic shots were fired over this sensors network using an air gun at variable water depth. Finally, the sensors were left recording on the seafloor for 28 days. The recorded seismic data allow to test the capacity of both active and passive seismic imaging process to assess the shear modulus in a 2D medium under seabed.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

2 - Site characterization, in-situ and laboratory testing, measurement